BTW there is a help and bugs forum for a reason. IIRC that is a mod error, but in this case it may not be. Try restarting your comp, try a normal map like altored divide or something like that to see if that fixes it.
You are starting spring.exe directly, or through a lobby with bots? What is the exact sequence that you are using?
It has nothing to do with anything that forb mentioned. Using task manager's process list, check for a spring.exe that is still open (and thus has an open listening socket on the port that the new one is trying to use) and kill it.